The present invention relates generally to a method for deriving compositional information. More specifically the invention relates to a method for determining the separate moisture contents and other compositional information for layered webs containing hydrogen bonding or hydrophilic and/or non-hydrogen bonding or hydrophobic layers, including but not restricted to photographic film or paper or their precursor support webs.
The measurement of moisture in moving webs, particularly in paper, has been the subject of a number of patents. See U.S. Pat. Nos. 3,405,268, Brunton; 3,551,678, Mitchell; 3,641,349, Dahlin; 3,675,019, Hill; 3,793,524, Howarth; 4,006,358, Howarth; 4,097,743, Carlson; 4,345,150, Tamura; 4,840,706, Campbell. These patents disclose methods and apparatuses that measure the intensity of transmitted or reflected light through a series of optical filters that transmit selectively in the near-infrared (NIR) region of the spectrum (0.8 to 2.5 micrometers). One filter passes light at a wavelength which is water absorbing, generally specified as 1.94 micrometers, while a second filter passes light at a wavelength that water does not absorb, typically 1.8 micrometers. Some patents further disclose use of an additional filter at a wavelength where the web (e.g., cellulose) absorbs light, and even a fourth filter at a wavelength where the web does not absorb light.
Attempts to use such a device to measure the moisture content of photographic film after drying have produced very noisy results and in general a poor correlation with other moisture measurement techniques. During research associated with the present invention, examination of the transmission infrared spectrum of photographic film webs revealed that the very small features associated with absorption by water in the NIR are superimposed on a very large background of light attenuation that is associated with light scattering by the silver halide particles in the emulsion layers of the film. This background attenuation increases at shorter wavelengths and is generally not reproducible in either absolute amount or in the amount of change with wavelength between films.
Background attenuation has two effects. First, the wavelengths at which water and web do not absorb have a large underlying absorbance (due to light scattering) relative to the additional absorbance of water or web at their respective measurement wavelengths. The absorbance due to light scattering results in a very small moisture signal relative to a very large, noisy background and thus a very noisy and unsatisfactory measurement. Second, the fact that the baseline changes with wavelength and that the change is different from coating to coating is not compensated for when a single reference wavelength is used for each measurement wavelength.
A further disadvantage of available NIR filter-type moisture measurement systems is that a single wavelength is used to obtain the measure of moisture. If water in the different layers of a web had exactly identical absorption properties as a function of wavelength, then the measurement outlined above would provide a signal proportional to the total amount of moisture in the entire web if the measurement were made with transmitted light. If the measurement is made with reflected or diffusely scattered light, the measurement signal would be proportional to the total amount of water in that fraction of the cross section of the web that light penetrated to before it was reflected or scattered. The prior art apparently assumes that water in different layers of a web has identical absorption properties as a function of wavelength.
During research associated with the present invention, it has been discovered that water contained in different chemical environments has different spectra in both the mid-infrared (2.5-14 micrometers) and the near infrared (0.8-2.5 micrometers) wavelength regions. These differing spectra indicate that the amount of light absorbed by a given amount of water at a given wavelength will depend on the environment in which the water is contained, and in particular whether the water is hydrogen bonded or not hydrogen bonded to the molecules in a given environment.
For photographic film and paper products the wavelength 1.94 micrometers is absorbed to a much greater extent by non-hydrogen bonded water in a hydrophobic environment (as in synthetic polymer supports such as cellulose acetate or polyethylene terephthalate, or in paper) than by the water that is hydrogen bonded as in the gelatin in the emulsion layers of the photographic product. These differing absorptions mean that the signal measured by simple NIR filter moisture devices is not a simple measurement of the total amount or fraction of water in the entire web. At the same time the desirable result of separate measures of water in support and of water in the emulsion layers cannot be obtained either.
A further disadvantage of filter-type infrared instruments for web measurement is that they require at least one measurement wavelength filter for each composition component measured (more typically an additional reference wavelength filter per component as well). In many web coating processes there is a large variation in the components of the support and coated layers so that it becomes very difficult to provide a filter-based infrared instrument with enough flexibility to measure the components of these varied coating processes because the number of filters required becomes too large to provide rapid feedback.
Hence, there is a need for a much more flexible system that would involve obtaining the entire spectrum (light intensity as a function of wavelength) in all cases and providing an analysis system that can recognize the product variations and select appropriate wavelengths to analyze the specific product being coated.
Another disadvantage of present measuring methods involves webs that are not completely dry. When taking the background spectrum through open air, present methods generally require that the moving web be stopped and removed. Taking a background spectrum over a stopped web does not accurately portray the components of the open air over a moving web.
In summary, the disadvantages of present filter-based NIR measurements for moving webs include (1) they provide noisy and inaccurate measurements in the case of webs that contain highly scattering media, (2) they provide inadequate selectivity to measure water in different chemical environments, (3) they provide inadequate flexibility for situations in which the components of the web change greatly in batch coating processes, and (4) they fail to account for open air composition over moving non-dry webs.
Vidrine (U.S. Pat. No. 4,943,721) discloses the combined use of an FTIR (Fourier transform infrared) thickness gauge and a beta gauge wherein the FTIR gauge is used to coarsely set parameters that improve the beta gauge thickness measurement. Huizinga and others disclose the use of mid-infrared FTIR measurements for monitoring of thin film coatings. See "Application of Infrared Spectroscopy to On-Line Monitoring of Thin Coatings"; J. S. Huizinga, E. Rudin and N. G. Constantine; presented at the Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, St. Louis, 1986; and "Comparison of Near-IR and Mid-IR Spectroscopic Techniques Used for On-Line Quality Control of Moving Webs"; J. S. Huizinga; presented at the Pittsburgh Conference, Chicago, 1991. U.S. Pat. No. 4,243,882 discloses a method for measuring the thicknesses of film layers by using infrared rays. The '882 patent method uses several sample wavelengths and a reference wavelength.